The present invention relates to a device for controlling an electrical load. The controlled electrical load particularly relates to an arrangement of light-emitting diodes, hereinafter called LEDs, wherein the electrical load must be supplied with a nearly constant operating current.
Constant current sources are preferably used for controlling an electrical load, especially LEDs, LED chains and/or LED arrays. Diverse arrangements of LEDs are known. Besides the parallel arrangement or matrix connection of LEDs, the possibility of series connection of LEDs is also known. In the series connection of LEDs, all LEDs are connected behind one another in a row; this connection is also called an LED chain. To operate this LED chain, a constant current is generated and conducted through the LEDs. A voltage that corresponds to the sum of the forward voltages of all LEDs then arises across the LEDs.
In order to achieve a constant luminous efficiency, the current that flows through the LEDs must be controlled temperature-dependent and be nearly constant. This is achieved in a well-known manner through pulse-width modulation of the supplied current. By means of pulse-width modulation, this modulated current is then used for the brightness control of the LED chain. The energy supply of the LEDs is accomplished by a step-up converter, for example.
An LED cluster arrangement, which is supplied with constant current, is known from DE 20 2007 011 973 U1. The LED cluster arrangement is controlled by pulse-width modulation.
DE 2006 059 355 A1 discloses a control device in a method for operating a series connection of light-emitting diodes.
DE 10 2005 058 484 A1 discloses a circuit arrangement and a method for operating at least one LED.
Voltage and current variations that stress the energy supply unit particularly arise during switching operations, such as switching on/off single LEDs connected in series. The forward voltage, which drops at the LED for a corresponding current, is based on the current-voltage characteristic of a light-emitting diode. A particular minimum voltage is thus first necessary for operation. The LED current is nearly negligible until this minimum voltage is reached, and the light emission is zero or nearly zero.
If the brightness of single LEDs in the series connection is to be influenced, this is accomplished by jumping the LEDs using a switch arranged in parallel to each LED or to an LED group. The switch is advantageously embodied in the form of a semi-conductor switch. The current then flows either through the LEDs whose parallel switch is open or through the closed switches. This switching principle allows the LEDs to be switched on and off as desired.
As long as the number of LEDs remains constant, i.e. a switching operation does not change the number of LEDs switched on, the output voltage that the voltage supply unit must provide will remain unchanged. However, changing the simultaneously driven LEDs presents a problem, because the output voltage needed to operate the new number of LEDs changes and the LED current thus breaks down. If one LED among the operated or already illuminating LEDs is now switched on or off, a considerable voltage peak and a current variation appears. When switching on an LED, a current break therefore occurs at first due to the lack of output voltage and then a voltage peak occurs due to the control response. The result is that the LEDs that are already switched on and illuminating at first become dark and flicker. This must be avoided through a suitable control.
It is therefore the object of the invention to provide a device that handles this problem.